Low temperature thermal conductor

ABSTRACT

A thermal conductor material having excellent heat transfer properties by obtaining high thermal conductivity even at low temperature of, for example, a liquid nitrogen temperature (77 K) or lower is to provide. A thermal conductor to be used at low temperature of 77 K or lower in the magnetic field of a magnetic flux density of 1 T or more, includes aluminum which has a purity of 99.999% by mass or more and also has the content of iron of 1 ppm by mass or less.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a thermal conductor which exhibitsexcellent conductivity at low temperature of, for example, 77 K orlower, especially at cryogenic temperatures of 20 K or lower; and moreparticularly to a thermal conductor which exhibits excellentconductivity even when used in a strong magnetic field of, for example,1 T or more.

2. Description of the Related Art

A superconducting magnet has been used in various fields, for example,MRIs (magnetic resonance imaging) for diagnosis, NMRs (nuclear magneticresonance) for analytical use or maglev trains. There have been used, asa superconducting magnet, low-temperature superconducting coils cooledto helium's boiling point of 4.2 K (Kelvin) using liquid helium, andhigh-temperature superconducting coils cooled to about 20 K by arefrigerator.

There is a need to use a thermal conductor which exhibits high thermalconductivity at low temperature of a boiling point of liquid nitrogen(77 K) or lower, especially cryogenic temperatures of 20 K or lower, soas to cool these superconducting coils efficiently and uniformly.

JP 2007-063671A discloses cold-worked aluminum, as a thermal conductorwhich exhibits high thermal conductivity at low temperature.

JP 2004-283580A discloses a structure of a magnetic resonance assembly,and also describes that it is possible to use, as a thermal conductor(thermal bus bar) located between a refrigerator and a freezingcontainer, aluminum having high purity of 99.999% by mass or more(hereinafter sometimes referred to as “5N” (five nines) and, in the masspercentage notation which indicates a purity, notation is sometimesperformed by placing “N” in the rear of the number of “9” which iscontinuous from the head, for example, purity of 99.9999% by mass ormore is sometimes referred to as “6N” (six nines)), which exhibits highheat transfer properties at cryogenic temperatures, or aluminum having apurity of 99.99% by mass or more (4N).

There is also known a thermal conductor using copper such as oxygen-freecopper having a purity of 99.99% by mass or more (4N), in addition toaluminum.

However, these materials having high heat transfer properties at lowtemperature also have a problem that the thermal conductivity decreasesin the vicinity of a superconducting coil (superconducting magnet), forexample, under a strong magnetic field where the magnetic field producedby the superconducting coil is 1 T or more, and thus high heat transferproperties cannot be obtained.

This problem is caused by the magnetoresistance effect. This effect isknown as a phenomenon in which electrical resistivity varies dependingon the external magnetic field.

It is known that copper shows a remarkable magnetoresistance effect andthe electrical resistivity at low temperature remarkably increases withincreasing magnetic field. It is known that aluminum also shows themagnetoresistance effect, although not comparable to copper, and thatcauses a remarkable increase in electrical resistivity at lowtemperature in the magnetic field.

In lots of metals including copper, aluminum and alloys thereof, theelectrical resistivity has a close relation with the thermalconductivity, and the thermal conductivity decreases when the electricalresistivity increases (conductivity decreases).

As a result, there was a problem that cooling efficiency of asuperconducting coil decreases as heat transfer properties of a thermalconductor to be used under a strong magnetic field deteriorate.

SUMMARY OF THE INVENTION

Thus, an object of the present invention is to provide a thermalconductor having excellent heat transfer properties by obtaining highthermal conductivity even at low temperature of, for example, a liquidnitrogen temperature (77 K) or lower, especially cryogenic temperaturesof 20 K or lower in a strong magnetic field of a magnetic flux densityof 1 T or more.

The present invention provides, in an aspect 1, a thermal conductor tobe used at low temperature(s) of 77 K or lower in the magnetic field ofa magnetic flux density of 1 T or more, including aluminum which has apurity of 99.999% by mass or more and also has the content of iron of 1ppm by mass or less.

The present inventors have found that even aluminum (Al) can remarkablysuppress the magnetoresistance effect by controlling the purity to99.999% by mass or more and also controlling the content of iron to 1ppm by mass or less. The thermal conductor made of such aluminum hashigh thermal conductivity and exhibits excellent heat transferproperties even when used at cryogenic temperatures of, for example, 77K or lower in a strong magnetic field of a magnetic flux density of 1 Tor more.

The present invention provides, in an aspect 2, the thermal conductoraccording to the aspect 1, wherein the aluminum has a purity of 99.9999%by mass or more.

The present invention provides, in an aspect 3, the thermal conductoraccording to the aspect 1, wherein the aluminum has a purity of99.99998% by mass or more.

The present invention provides, in an aspect 4, the thermal conductoraccording to any one of the aspects 1 to 3, wherein the aluminumcontains an intermetallic compound Al₃Fe.

The present invention provides, in an aspect 5, the thermal conductorfor cooling a superconducting magnet, using the thermal conductoraccording to any one of the aspects 1 to 4.

According to the present invention, it is possible to provide a thermalconductor having excellent heat transfer properties by having highthermal conductivity even at low temperature of, for example, a liquidnitrogen temperature (77 K) or lower, especially cryogenic temperaturesof 20 K or lower in a strong magnetic field of a magnetic flux densityof 1 T or more.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a relation between the conductivity index andthe applied magnetic field (magnetic flux density).

FIG. 2 is a graph showing a relation between the thermal conductivityand the applied magnet field (magnetic flux density).

FIG. 3 is a graph showing a relation between the temperature differenceof both ends of a sheet-shaped sample and the magnetic field (magneticflux density).

DETAILED DESCRIPTION OF THE INVENTION

The thermal conductor according to the present invention includesaluminum which has a purity of 99.999% by mass or more and also has thecontent of iron of 1 ppm by mass, so as to be used even in the magneticfield of a magnetic flux density of 1 T or more.

The present inventors have found, first, that aluminum, which has apurity of 99.999% by mass or more and also has the content of iron of 1ppm by mass, does not remarkably exert the magnetoresistance effect evenwhen the magnetic field of a magnetic flux density of 1 T or more isapplied, and thus suppressing a decrease in thermal conductivity.Consequently, the present invention has been completed.

As disclosed, for example, in JP 2009-242865A and JP 2009-242866A, ithas been known that the electrical resistivity at cryogenictemperatures, for example, liquid helium temperatures decreases as thepurity of aluminum increases, like 5N (purity of 99.999% by mass ormore) and 6N (purity of 99.9999% by mass or more).

As disclosed, for example, in JP 2010-106329A, aluminum having a purityof 99.999% by mass or more and also having the content of iron of 1 ppmby mass or less has also been known.

It has been known that although aluminum enables an improvement inelectrical conductivity at cryogenic temperatures in a state where themagnetic field is not applied by increasing the purity to about 4N,remarkable magnetoresistance effect appears when a strong magnetic fieldof a magnetic flux density of 1 T or more is applied, and thus causing adecrease in conductivity It has been considered that high conductivitycannot be obtained under a strong magnetic field also in high purityaluminum of 5N or 6N purity, similarly to the aluminum of 4N purity.

Therefore, it is considered that aluminum having a purity of 99.999% bymass or more and also having the content of iron of 1 ppm by mass orless was not used in a thermal conductor which is used in the magneticfield of a magnetic flux density of 1 T or more.

It is as mentioned above that the present inventors have found, first,that an increase in electrical resistivity (i.e., a decrease in thermalconductivity) under a strong magnetic field, which has conventionallybeen conceived, does not occur in high purity aluminum of 5N or higherlevel and also having the content of iron of 1 ppm by mass or less.

Although details will be described in the below-mentioned examples, adrastic decrease in conductivity is recognized in a strong magneticfield even in a high purity copper of 5N, 6N or higher level purity,although this material is commonly used as a thermal conductor.Therefore, a phenomenon in which high conductivity is maintained even ina strong magnetic field by achieving high purity, found by the presentinventors, is peculiar to aluminum.

In the thermal conductor according to the present invention, asmentioned above, the amount of iron contained in aluminum is controlledto 1 ppm by mass or less.

As will be described below for details, the reason is considered asfollows: the magnetoresistance effect is surely suppressed bycontrolling the amount of iron as a ferromagnetic element, and thusmaking it possible to surely suppress a decrease in thermal conductivitycaused by the applied strong magnetic field.

The thermal conductor according to the present invention remarkablyexerts the effect by use in a state where the temperature is 77 K (−196°C.) or lower, and more preferably 20 K (−253° C.) or lower, and also themagnetic field of a magnetic flux density of 1 T or more is applied.

Before making a description of details of the thermal conductoraccording to the present invention, a description is made why a thermalconductor using a material having excellent electrical conductivity hashigh thermal conductivity.

In lots of metals including copper, aluminum and alloys thereof,transfer of free electrons is the main mechanism of electric conductionand the electrical conductivity can be enhanced by making free electronsto easily transfer. On the other hand, free electrons significantlycontribute to thermal conduction of these metals, and high thermalconductivity can be obtained when free electrons are easily movable.

Wiedemann-Franz (WF) law has been known as a relation between thethermal conductivity and the electrical conductivity of common metals.It has also been known that the thermal conductivity of about 40 K orlower of high purity aluminum can be determined from the followingequation (1) as a more accurate relational equation of high puritymetals, and the thermal conductivity of about 40 K or lower of highpurity copper can be determined from the following equation (2) (bothequations are cited from TEION KOGAKU, Vol. 39 (2004), No. 1, pp.25-32).

κ=1/(1.83×10⁻⁷ ×T ²+1.09/RRR/T)  (1)

κ=1/(6.41×10⁻⁸ ×T ^(2.4)+0.685/RRR/T)  (2)

where

κ: Thermal conductivity (W/m/K)

T: Temperature (K)

RRR: Residual resistivity ratio

The residual resistivity ratio RRR is represented by the followingequation (3).

RRR=ρ _(297 K)/ρ_(T)  (3)

where

ρ_(297 K): Resistivity at temperature of 297 K (nΩcm)

ρ_(T): Resistivity at temperature T (K) (nΩcm)

Herein, it has been known that ρ_(297 K) of copper and ρ_(297 K) ofaluminum are scarcely influenced by the purity and the magnetic field tobe applied from the outside, and are almost constant (for example,ρ_(297 K) of aluminum is about 2,700 and ρ_(297 K) of copper is about1,500).

Therefore, as is apparent from the equations (1) to (3), the thermalconductivity of copper and aluminum increases as the electricalconductivity is improved (as the electrical resistivity decreases).

Details of the thermal conductor according to the present invention willbe described below.

(1) Level of Impurities

As mentioned above, the thermal conductor according to the presentinvention is characterized by including aluminum which has a purity of99.999% by mass or more and also has the content of iron of 1 ppm bymass or less. The purity is preferably 99.9999% by mass or more, andmore preferably 99.99998% by mass or more (hereinafter sometimesreferred to as “6N8”) for the following reasons. That is, the higher thepurity, the less the decrease in electrical conductivity under a strongmagnetic field. Furthermore, in case of the purity of 99.9999% by massor more, the electrical resistivity may sometimes decrease in a strongmagnetic field of 1 T or more as compared with the case where themagnetic field is not applied.

The content of iron is preferably 0.1 ppm by mass or less.

The reason is that a decrease in conductivity in a strong magnetic fieldcan be suppressed more surely.

There are still many unclear points in the mechanism in which a decreasein electrical conductivity in a strong magnetic field can be suppressedby controlling the content of iron to 1 ppm by mass or less. However,predictable mechanism at the moment is considered as follows. That is,iron is likely to be influenced by a strong magnetic field since it is aferromagnetic element and, as a result, when iron exists in the contentof more than 1 ppm by mass, an influence exerted on the conductivityincreases, and thus the conductivity under a strong magnetic field maydecrease. When the content of iron is 0.1 ppm, an influence due to theferromagnetic material can be excluded almost completely. However, thispredictable mechanism does not limit the scope of the present invention.

Ni and Co are known as ferromagnetic elements other than iron. However,since these elements are easily removed in a known process for highlypurification of aluminum, the numerical value of the content is out ofthe question. However, the contents of these Ni and Co are alsopreferably 1 ppm or less, and more preferably 0.1 ppm or less.

The purity of aluminum can be defined in some methods. For example, itmay be determined by the measurement of the content of aluminum.However, it is preferred that the purity of aluminum is determined bymeasuring the content (% by mass) of the following 33 elements containedas impurities in aluminum and subtracting the total of these contentsfrom 100%, so as to determine the purity of aluminum with high accuracyin a comparatively simple manner.

Herein, 33 elements contained as impurities are lithium (Li), beryllium(Be), boron (B), sodium (Na), magnesium (Mg), silicon (Si), potassium(K), calcium (Ca), titanium (Ti), vanadium (V), chromium (Cr), manganese(Mn), iron (Fe), nickel (Ni), cobalt (Co), copper (Cu), zinc (Zn),gallium (Ga), arsenic (As), zirconium (Zr), molybdenum (Mo), silver(Ag), cadmium (Cd), indium (In), tin (Sn), antimony (Sb), barium (Ba),lantern (La), cerium (Ce), platinum (Pt), mercury (Hg), lead (Pb) andbismuth (Bi).

The contents of these elements can be determined, for example, by glowdischarge mass spectrometry.

(2) Purification Method

Such high purity aluminum may be obtained by using any purification(refinement) method. Some purification methods for obtaining high purityaluminum according to the present invention are exemplified below.However, the purification method is not limited to these methods as amatter of course.

Three-Layer Electrolysis Process

It is possible to use, as one of methods of obtaining high purityaluminum, a three-layer electrolysis process in which commerciallyavailable aluminum having comparatively low purity (for example withspecial grade 1 of 99.9% purity as specified in JIS-H2102) is charged inan Al—Cu alloy layer and is used as an anode in a molten state, and anelectrolytic bath containing aluminum fluoride and barium fluoridetherein is arranged thereon, and thus high purity aluminum is producedon a cathode.

In the three-layer electrolysis process, aluminum having a purity of99.999% by mass or more can be mainly obtained. It is possible tosuppress the content of iron in aluminum to 1 ppm by mass or less,comparatively easily.

Unidirectional Solidification Process

For example, a unidirectional solidification process can be used so asto further increase a purity of the high purity aluminum obtained by thethree-layer electrolysis process.

The content of Fe and the respective contents of Ti, V, Cr and Zr can beselectively decreased by the unidirectional solidification process.

It has been known that the unidirectional solidification process is, forexample, a method in which aluminum is melted in a furnace tube using afurnace body moving type tubular furnace and then unidirectionallysolidified from the end by pulling out a furnace body from a furnacetube, and that the contents of the respective elements of Ti, V, Cr andZr selectively increase at the side of the solidification initiationend, and also the content of Fe selectively increases at the side of thesolidification completion end (opposite side of the solidificationinitiation end). Therefore, it becomes possible to surely decrease thecontents of the respective elements of Fe, and Ti, V, Cr and Zr bycutting off the both sides of solidification initiation end and thesolidification completion end of the obtained ingot. It may bedetermined, which specific portion of the ingot obtained by theunidirectional solidification process must be cut, by analyzing thecontents of elements at appropriate intervals along a solidificationdirection so that only portion, where the total content of the contentsof Fe, and Ti, V, Cr and Zr is sufficiently decreased, is allowed toremain.

There is no particular limitation on the order of implementation ofpurification by the three-layer electrolysis process and purification bythe unidirectional solidification process. Usually, purification isimplemented by the three-layer electrolysis process, and thenpurification is implemented by the unidirectional solidificationprocess. Purification by the three-layer electrolysis process andpurification by the unidirectional solidification process may beimplemented, for example, alternately and repeatedly, or any one of orboth purifications may be repeatedly implemented, respectively. It isparticularly preferred that purification by the unidirectionalsolidification process is repeatedly implemented.

In such way, aluminum having a purity of 99.9999% by mass or more can beobtained by using the three-layer electrolysis process in combinationwith the unidirectional solidification process. It is also possible tosuppress the content of iron in aluminum to 1 ppm by mass or less, andmore preferably 0.1 ppm by mass or less in a comparatively easy manner.

Zone Melting Process

Furthermore, a zone melting process can be used so as to obtain aluminumhaving high purity, for example, a purity of 99.99998% by mass or more.When the zone melting process is appropriately used, the content of ironin aluminum can be suppressed to 1 ppm by mass or less, and morepreferably 0.1 ppm by mass or less, more surely.

In particular, it is effective to use a purification method of aluminumthrough the zone melting process invented by the present inventors(method described in Japanese Patent Application No. 2010-064544. Thedisclosure of Japanese Patent Application No. 2010-064544 isincorporated by reference herein.).

In order to prevent impurities from diffusing into heated aluminum whenremoving impurities in aluminum through zone melting purificationprocess, it is preferred that an alumina layer is formed in advance on asurface of a boat in which aluminum is placed, and also zone meltingpurification is performed in vacuum under a pressure of 3×10⁻⁵ Pa orless, and more preferably from 3×10⁻⁶ Pa to 2×10⁻⁵ Pa, so as to surelyseparate impurities from molten aluminum.

It is preferred to carry out a pretreatment, in which a surface layer ofan aluminum raw material to be subjected to zone melting purification isdissolved and removed in advance, before zone melting purification isperformed. There is no particular limitation of the pretreatment method,and various treatments used in the relevant technical field can be usedso as to remove the surface layer of the aluminum raw material.

Examples of the pretreatment include an acid treatment, an electrolyticpolishing treatment and the like.

The above-mentioned boat to be used in the zone melting purificationprocess is preferably a graphite boat, and is preferably baked in aninert gas or vacuum after formation of the above-mentioned aluminalayer.

The width of the melting section where aluminum is melted during thezone melting purification is preferably adjusted to w_(Al)×1.5 or moreand w_(Al)×6 or less based on a cross sectional size w_(Al) of thealuminum raw material.

An aluminum raw material to be used in the purification is obtained byusing the three-layer electrolysis process in combination with theunidirectional solidification process and, for example, high purityaluminum having a purity of 99.9999% by mass or more is preferably used.

In the zone melting process, for example, the melting section is movedfrom one end of a raw aluminum toward the other end by moving a highfrequency coil for high frequency heating, and thus the entire rawaluminum can be subjected to zone melting purification. Among impuritymetal element components, peritectic components (Ti, V, Cr, As, Se, Zrand Mo) tend to be concentrated to the melting initiation section andeutectic components (26 elements as a result of removal of peritectic 7elements from the above-mentioned 33 impurity elements) tend to beconcentrated to the melting completion section, and thus a high purityaluminum can be obtained in the region where both ends of the aluminumraw material are removed.

After moving the melting section within a predetermined distance, like adistance from one end to the other end in a longitudinal direction of analuminum raw material, high frequency heating is completed and themelting section is solidified. After the solidification, an aluminummaterial is cut out (for example, both ends are cut off) to obtain apurified high purity aluminum material.

When a plurality of aluminum raw materials are arranged in alongitudinal direction (in a movement direction of the melting section),it is preferred that the aluminum raw materials in a longitudinaldirection are brought into contact with each other to treat as onealuminum raw material in a longitudinal direction, and then the meltingsection is moved from one end (i.e., one of two ends where adjacentaluminum raw materials are not present in a longitudinal direction amongends of the plurality of aluminum raw materials) to the other end (i.e.,the other one of two ends where adjacent aluminum raw materials are notpresent in a longitudinal direction among ends of the plurality ofaluminum raw materials).

The reason is that ends of the aluminum raw material contacted with eachother are united during zone melting, and thus a long aluminum materialcan be obtained.

As mentioned above, after zone melting (zone melting purification) fromone end to the other end of the aluminum raw material, zone melting canbe repeated again from one end to the other end. The number of repeattimes (number of passes) is usually 1 or more and 20 or less. Even ifthe number of passes is more than the above range, an improvement in thepurification effect is restrictive.

In order to effectively remove the peritectic 7 elements, the number ofpasses is preferably 3 or more, and more preferably 5 or more. When thenumber of passes is less than the above range, peritectic 7 elements areless likely to moved, and thus sufficient purification effect is notobtained.

The reason is as follows. When a plurality of aluminum raw materials arearranged in contact with each other in a longitudinal direction, whenthe number of passes is less than 3, a shape (especially, height size)of the purified aluminum after uniting becomes un-uniform, and thus themelting width may sometimes vary during purification and uniformpurification is less likely to be obtained.

(3) Forming Method

The ingot of the high purity aluminum obtained by the above-mentionedpurification method is formed into a desired shape using variousmethods.

The forming method will be shown below. However, the forming method isnot limited thereto.

Rolling

When a thermal conductor to be obtained is a plate or a wire, rolling isan effective forming method.

The rolling may be performed using a conventional method, for example, amethod in which an ingot is passed through a pair of rolls byinterposing into the space between these rolls while applying apressure. There is no particular limitation on specific techniques andconditions (temperature of materials and rolls, treatment time,reduction ratio, etc.) in case of rolling, and these specific techniquesand conditions may be appropriately set unless the effects of thepresent invention are impaired.

There is no particular limitation on the size of the plate and wire tobe finally obtained by rolling. As for preferable size, the thickness isfrom 0.1 mm to 3 mm in case of the plate, or the diameter is from 0.1 mmto 3 mm in case of the wire.

When the thickness is less than 0.1 mm, sufficient conductioncharacteristics required as the thermal conductor may be sometimes lesslikely to be obtained since a cross section decreases. In contrast, whenthe thickness is more than 3 mm, it may sometimes become difficult todeform utilizing flexibility. When the thickness is from 0.1 mm to 3 mm,there is an advantage such as easy handling, for example, and thematerial can be arranged on a side surface of a curved containerutilizing flexibility.

As a matter of course, the shape obtainable by rolling is not limited tothe plate or wire and, for example, a pipe shape and an H-shape can beobtained by rolling.

The rolling may be hot rolling or warm rolling in which an ingot isheated in advance and then rolling is performed in a state of being setat a temperature higher than room temperature, or may be cold rolling inwhich the ingot is not heated in advance. Alternatively, hot rolling orwarm rolling may be used in combination with cold rolling.

In case of rolling, it is also possible to cast or cut the material intoa desired shape in advance. In case of casting, a conventional methodmay be employed, but is not limited to, for example, a method in whichhigh purity aluminum is heated and melted to form a molten metal and theobtained high purity aluminum molten metal is solidified by cooling in amold. Also, there is no particular limitation on the conditions or thelike in case of casting. The heating temperature is usually from 700 to800° C., and heating and melting is usually performed in vacuum or aninert gas (nitrogen gas, argon gas, etc.) atmosphere in a crucible suchas a graphite crucible.

Forming Method Other than Rolling

Wire Drawing or extrusion may be performed as a forming method otherthan rolling. There is no limitation on the shape obtained by drawing orextrusion. For example, drawing or extrusion is suited to obtain a wirehaving a circular cross section.

A desired wire shape may be obtained by rolling before drawing to obtaina rolled wire (rolled wire rod) and then drawing the rolled wire.

The cross section of the obtained wire is not limited to a circle andthe wire may have a noncircular cross section, for example, an oval orsquare cross section.

The desired shape may also be obtained by cutting the ingot, except fordrawing or extrusion.

(4) Annealing

Furthermore, the formed article of the present invention obtained by theabove forming method such as rolling may be optionally subjected to anannealing treatment. It is possible to remove strain, which may beusually sometimes generated in case of cutting out a material to beformed from the ingot, or forming, by subjecting to an annealingtreatment.

There is no particular limitation on the conditions of the annealingtreatment, and a method of maintaining at 400 to 600° C. for one or morehours is preferable.

When the temperature is lower than 400° C., strain (dislocation)included in the ingot is not sufficiently decreased for the followingreason. Since strain (dislocation) serves as a factor for enhancingelectrical resistivity, excellent conduction characteristics may not besometimes exhibited. When the heat treatment temperature is higher than600° C., solution of impurities in solid, especially solution of ironinto a matrix proceeds. Since solid-soluted iron has large effect ofenhancing electrical resistivity, conduction characteristics maysometimes deteriorate.

More preferably, the temperature is maintained at 430 to 550° C. for oneor more hours for the following reason.

When the temperature is within the above range, strain can besufficiently removed and also iron exists as an intermetallic compoundwith aluminum without being solid-soluted into the matrix.

The following reasons are also exemplified.

As an intermetallic compound of iron and aluminum, for example, aplurality of kinds such as Al₆Fe, Al₃Fe and Al_(m)Fe (m≈4.5) are known.It is considered that the majority of (for example, 50% or more, andpreferably 70% or more in terms of volume ratio) of an intermetalliccompound of iron and aluminum, which exists in a high purity aluminummaterial obtained after annealing within a temperature range (430 to550° C.), is Al₃Fe.

This Al₃Fe has such an advantage that it scarcely exerts an adverseinfluence on the conductivity even in case of existing as a precipitate.

Existence of Al₃Fe and the volume ratio thereof can be confirmed andmeasured by dissolution of a matrix (base material) using a chemicalsolvent, and collection by filtration, followed by observation of theresidue collected by filtration using an analytical electron microscope(analytical TEM) and further analysis.

The thermal conductor according to the present invention may be composedonly of the above-mentioned high purity aluminum having a purity of99.999% by mass or more and may contain the portion other than the highpurity aluminum, for example, protective coating so as to impart variousfunctions.

While a thermal conductor for cooling a superconducting magnet isillustrated as specific applications of the thermal conductor accordingto the present invention, the specific application is not limitedthereto and the thermal conductor according to the present invention canbe used as thermal conductors for various applications used at lowtemperature (77 K or lower) under a strong magnetic field (1 T or more),for example, thermal conductors used for cooling specimens to bemeasured in NMR.

EXAMPLES

Example 1 (purity of 99.999% by mass or more, 5N—Al), Example 2 (purityof 99.9999% by mass or more, 6N—Al) and Example 3 (purity of 99.99998%by mass or more, 6N8-Al), details of which are shown below, wereproduced as example samples, and then resistivity (specific electricalresistivity) was measured.

Comparative Example 1 (4N—Al) as aluminum having a purity of 4N level,and Comparative Example 2 (3N—Al) as aluminum having a purity of 3Nlevel are shown below as Comparative Examples. The resistivity ofComparative Examples 1 and 2 was determined by calculation.

For comparison with copper, a sample of copper having a purity of 5Nlevel was prepared and then the resistivity was measured as ComparativeExample 3.

As for copper, literature data were used as Comparative Example.Comparative Example 4 is copper sample having a purity of 4N level,Comparative Example 5 is copper sample having a purity of 5N level, andComparative Example 6 is copper sample having a purity of 6N level.

(1) Production of High Purity Aluminum

First, the method for producing a high purity aluminum used in Examples1 to 3 is shown below.

Example 1

A commercially available aluminum having a purity of 99.92% by mass waspurified by the three-layer electrolysis process to obtain a high purityaluminum having a purity 99.999% by mass or more and an iron content of1 ppm by mass or less.

Specifically, a commercially available aluminum (99.92% by mass) wascharged in an Al—Cu alloy layer and the composition of an electrolyticbath was adjusted to 41% AlF₃-35% BaF₂-14% CaF₂-10% NaF. Electricity wassupplied at 760° C. and a high purity aluminum deposited at a cathodeside was collected.

The contents of the respective elements in this high purity aluminumwere analyzed by glow discharge mass spectrometry (using “VG9000”,manufactured by THERMO ELECTRON Co., Ltd) to obtain the results shown inTable 1.

Example 2

The high purity aluminum obtained by the above-mentioned three-layerelectrolysis process was purified by the unidirectional solidificationto obtain a high purity aluminum having a purity 99.9999% by mass ormore and an iron content of 1 ppm by mass or less.

Specifically, 2 kg of the high purity aluminum obtained by thethree-layer electrolysis process was placed in a crucible (insidedimension: 65 mm in with×400 mm in length×35 mm in height) and thecrucible was accommodated inside a furnace tube (made of quartz, 100 mmin inside diameter×1,000 mm in length) of a furnace body transfer typetubular furnace. The high purity aluminum was melted by controlling afurnace body (crucible) to 700° C. in a vacuum atmosphere of 1×10⁻² Pa,and then unidirectionally solidified from the end by pulling out thefurnace body from the furnace tube at a speed of 30 mm/hour. Aftercutting out from the position which is 50 mm from the solidificationinitiation end in a length direction to the position which is 150 mmfrom the solidification initiation end, a massive high purity aluminummeasuring 65 mm in width×100 mm in length×30 mm in thickness wasobtained.

The contents of the respective elements in this high purity aluminumwere analyzed by glow discharge mass spectrometry in the same manner asdescribed above to obtain the results as shown in Table 1.

Example 3

A high purity aluminum having a purity of 99.99998% by mass or more andthe iron content of 0.1 ppm or less was obtained by the following zonemelting process.

After cutting into a quadrangular prism measuring about 18 mm×18 mm×100mm or a similar shape from the 6N aluminum ingot obtained by theabove-mentioned unidirectional solidification process, and further acidpickling with an aqueous 20% hydrochloric acid solution prepared bydiluting with pure water for 3 hours, an aluminum raw material wasobtained.

Using this aluminum raw material, a zone melting process was carried outby the following method.

A graphite boat was placed inside a vacuum chamber (a quartz tubemeasuring 50 mm in outside diameter, 46 mm in inside diameter, 1,400 mmin length) of a zone melting purification apparatus. A high purityalumina powder AKP Series (purity: 99.99%) manufactured by SumitomoChemical Company, Limited was applied to the portion, where the rawmaterial is placed, of the graphite boat while pressing to form analumina layer.

The graphite boat was baked by high frequency heating under vacuum.

The baking was carried out by heating in vacuum of 10⁻⁵ to 10⁻⁷ Pa usinga high frequency heating coil (heating coil winding number: 3, 70 mm ininside diameter, frequency of about 100 kHz) used in zone melting, andmoving from one end to the other end of the boat at a speed of 100mm/hour thereby sequentially heating the entire graphite boat.

The above-mentioned 9 aluminum raw materials in total weight of about780 g were arranged on the portion (measuring 20 mm×20 mm×1,000 mm),where the raw materials are placed, provided in the graphite boat. Thealuminum raw materials were arranged in the form of a quadrangular prismconsisting of 9 raw materials (cross sectional size w of aluminum rawmaterials=18 mm, length L=900 mm, i.e. L=w×50).

After sealing inside a chamber, evacuation was carried out by aturbo-molecular pump and an oil sealed rotary pump until the pressurereaches 1×10⁻⁵ Pa or less. Then, one end of the aluminum raw material ina longitudinal direction was heated and melted using a high frequencyheating coil (high frequency coil) to form a melting section.

The output of the high frequency power source (frequency: 100 kHz,maximum output: 5 kW) was adjusted so that the melting width of themelting section becomes about 70 mm. Then, the high frequency coil wasmoved at a speed of 100 mm per hour thereby moving the melting sectionby about 900 mm. At this time, the pressure in the chamber was from5×10⁻⁶ to 9×10⁻⁶ Pa. The temperature of the melting section was measuredby a radiation thermometer. As a result, it was from 660° C. to 800° C.

Then, high frequency output was gradually decreased thereby solidifyingthe melting section.

The high frequency coil was moved to the melting initiation position(position where the melting section was formed first) and the aluminumraw material was heated and melted again at the melting initiationposition to form a melting section while maintaining vacuum inside thechamber. Zone melting purification was repeated by moving this meltingsection. At the moment when zone melting purification was carried outthree times (3 passes) in total at a melting width of about 70 mm and atraveling speed of 100 mm/hour of the melting section, the shape fromthe melting initiation section to the completion section became almostuniform, and uniform shape was maintained from then on (during 7 passesmentioned below).

Then, zone melting purification was carried out 7 passes at a meltingwidth of about 50 mm and a traveling speed of 60 mm/hour of the meltingsection. The melting width was from w×2.8 to w×3.9 based on a crosssectional size w of the aluminum raw material to be purified.

After completion of 10 passes in total, the chamber was opened toatmospheric air and then aluminum was removed to obtain a purifiedaluminum of about 950 mm in length.

The obtained aluminum was cut out and glow discharge mass spectrometrycomponent analysis was carried out in the same manner as describedabove. The results are shown in Table 1.

TABLE 1 Unit: ppm by mass Comparative Comparative Example 1 Example 2Example 1 Example 1 Example 3 Li 0.016 <0.001 <0.001 <0.001 Be 0.042<0.001 <0.001 <0.001 B 1.5 2.8 0.019 0.007 0.001 Na 1.4 0.012 0.0010.001 Mg 5.2 0.1 0.48 0.10 0.001 Si 200 25 2.3 0.34 0.003 K <0.001 0.0130.008 0.008 Ca 1.3 0.002 0.002 0.003 Ti 29 0.7 0.060 0.027 0.031 V 532.2 0.023 0.027 0.023 Cr 3.9 2.1 0.025 0.026 0.022 Mn 2.1 2.1 0.0070.004 0.006 Fe 230 12 0.60 0.089 0.001 Ni 0.19 0.018 0.004 0.001 Co 130.3 <0.001 <0.001 <0.001 Cu 0.72 1 1.1 0.14 0.016 Zn 13 7 0.22 0.0020.001 Ga 93 12 0.006 0.001 0.001 As 0.023 0.029 0.001 0.001 Zr 4.8 0.0230.030 0.036 Mo 0.35 <0.001 <0.003 <0.004 Ag 1.1 <0.001 <0.001 <0.001 Cd<0.001 0.002 0.002 0.002 In 0.009 <0.001 <0.001 <0.001 Sn 1.1 0.0010.001 0.002 Sb 0.001 <0.001 <0.001 <0.001 Ba 0.004 <0.001 <0.001 <0.001La 0.038 0.045 0.001 0.001 Ce 0.095 0.17 0.001 0.001 Pt <0.001 0.0020.001 0.001 Hg <0.001 0.001 0.003 0.002 Pb 1.9 0.004 0.001 0.001 Bi<0.001 0.001 0.001 0.001 Total 669 67 <5.4 <8.3 <0.18

Then, the thus obtained high purity aluminum of Examples 1 to 3 wererespectively cut to obtain materials for wire drawing each measuring 6mm in width×6 mm in thickness×100 mm in length. In order to removecontamination elements due to cutting of a surface of the material forwire drawing, acid pickling was performed using an acid prepared at aratio (hydrochloric acid:pure water=1:1) for 1 hour, followed by washedwith running water for more than 30 minutes.

The obtained material for wire drawing was drawn to a diameter of 0.5 mmby rolling using grooved rolls and wire drawing. The specimen obtainedby wire drawing was fixed to a quartz jig, maintained in vacuum at 500°C. for 3 hours and then furnace-cooled to obtain a sample for theresistivity measurement.

Furthermore, a commercially available high purity copper having a purityof 5N level (manufactured by NewMet Koch, 99.999% Cu, 0.5 mm indiameter) as the sample of Comparative Example 3 was fixed to a quartzjig, washed with an organic solvent, maintained in vacuum at 500° C. for3 hours and then furnace-cooled to obtain a sample for the resistivitymeasurement.

(2) Derivation of Resistivity Measurement of Resistivity

With respect to the samples of Examples 1 to 3 and Comparative Example3, the resistivity was actually measured.

After immersing the obtained sample in liquid helium (4.2 K), theresistivity was measured by varying the magnetic field to be applied tothe sample from a magnetic flux density 0 T (magnetic field was notapplied) to 15 T, using the four wire method.

The magnetic field was applied in a direction parallel to a longitudinaldirection of the sample.

Calculation of Resistivity

With respect to Comparative Example 1 and Comparative Example 2 with thecomposition shown in Table 1, calculation was performed using thefollowing equation (4) disclosed in the literature: R. J. Corruccini,NBS Technical Note, 218 (1964). In the equation (4), Δρ_(H) is an amountof an increase in resistivity in the magnetic field. ρ_(RT) isresistivity at room temperature when the magnetic field is not applied,and was set to 2,753 nΩcm since it can be treated as a nearly givenvalue in high purity aluminum having a purity of 3N or more. ρ isresistivity at 4.2 K when the magnetic field is not applied and largelyvaried depending on the purity. Therefore, the following experimentalvalues were used: 9.42 nΩcm (RRR=285) in 4N—Al and 117 nΩcm (RRR=23) in3N—Al. These equations are obtained in case the magnetic field isperpendicular to a longitudinal direction of the sample. However, sincesimilar equations in case the magnetic field is parallel to alongitudinal direction of the sample are not obtained, these equationswere used for comparison. RRR is also called a residual resistivityratio and is a ratio of resistivity at 297 K to resistivity at a heliumtemperature (4.2 K).

$\begin{matrix}{\frac{{\Delta\rho}_{H}}{\rho} = \frac{H_{*}^{2}\left( {1 + {0.00177\; H_{*}}} \right)}{\left( {1.8 + {1.6\; H_{*}} + {0.53\; H_{*}^{2}}} \right)}} & (4)\end{matrix}$

where

H*=H/100ρ_(RT)/ρ_(R)

H=Intensity of applied magnetic field (Tesla)

ρ_(RT)=Resistivity at room temperature when magnetic field is notapplied

ρ=Resistivity when magnetic field is not applied

Citation from Literatures relating to Resistivity

With respect to Comparative Examples 4 to 6, the resistivity wasobtained from the literature: Fujiwara S. et. al., Int. Conf. Process.Mater. Prop., 1st (1993), 909-912. In these literature data, a relationbetween the application direction of the magnetic field and thelongitudinal direction of the sample is not described.

The thus derived values of resistivity of Examples 1 to 3 andComparative Examples 1 to 6 are shown in Table 2.

TABLE 2 Resistivity ρ (nΩcm) 0 T 1 T 2 T 3 T 4 T 6 T 8 T 10 T 12 T 15 TExample 3 0.333 0.260 0.261 0.246 0.253 0.260 0.249 0.254 0.268 0.286Example 2 0.353 0.294 0.292 0.298 0.297 0.303 0.307 0.318 0.328 0.384Example 1 0.72 1.04 1.02 1.05 1.02 1.02 1.06 1.06 1.05 1.06 Comparative9.42 16.8 20.5 22.4 23.6 24.9 25.7 26.3 26.8 27.3 Example 2 Comparative117 120 127 135 144 163 179 194 206 221 Example 1 Comparative 1.57 3.584.73 5.4 5.8 6.4 6.7 7.0 7.2 7.4 Example 3 Comparative 3 6.1 10 13 17 2228 35 41 53 Example 6 Comparative 3.3 7.3 11 14 18 24 30.5 35 41 53Example 5 Comparative 4.6 9 13 17 21 28 34 40 46 56.5 Example 4

As is apparent from Table 2, in the sample of Comparative Example 2corresponding to a thermal conductor made of a conventional aluminum (4Nlevel), the resistivity increases as the intensity of the magnetic field(magnetic flux density) increases as compared with the case where themagnetic field is absent (0 T), and the resistivity increases by about 3times at 15 T.

To the contrary, in Examples 1 to 3, the resistivity is small such as atenth or less as compared with Comparative Example 2 in a state wherethe magnetic field is absent, and also the resistivity increase isslight even if the magnetic field increases.

In Example 1 (5N level), the resistivity at 15 T slightly increases(about 1.5 times) as compared with the case where the magnetic field isabsent, and it is apparent that the increase of the resistivity causedby magnetic field is small compared with Comparative Example 2.

In Example 2 (6N level), the resistivity slightly increases (within 10%)even at 15 T as compared with the case where the magnetic field isabsent. When the magnetic flux density is within a range from 1 to 12 T,the value of the resistivity decreased as compared with the case wherethe magnetic field is not applied, and thus remarkable magnetoresistancesuppression effect is exhibited.

As for Example 3 (6N8 level), the resistivity decreases as compared withthe case where the magnetic field is absent even at any magnetic fluxdensity of 1 to 15 T, and thus remarkable magnetoresistance suppressioneffect is exhibited.

FIG. 1 is a graph showing a relation between the electrical conductivityindex and the applied magnetic field (magnetic flux density). Theelectrical conductivity index is an index which indicates the magnitudeof the electrical conductivity of the respective samples based onComparative Example 2 which exhibits the resistivity in a strongmagnetic field of aluminum having a purity of 4N. Namely, in eachmagnetic flux density the electrical conductivity index is determined bydividing the value of the resistivity of Comparative Example 2 with thevalue of the resistivity of each sample. The larger the value of thisindex, the superior the conductive properties under the magnetic fluxdensity is compared with the sample of Comparative Example 2.

The electrical conductivity index of the ordinate was indicated bylogarithm since samples of Examples exhibit extremely remarkable effect.

As is apparent from FIG. 1, samples of Examples show the conductivity isabout 13 to 28 times higher than that of Comparative Example 2 even inthe case where the magnetic field is absent. As the magnetic field isapplied, the conductivity compared with Comparative Example 2 increases.The conductivity is 16 times (Example 1) to 65 times (Example 3) higherat 1 T, and the conductivity further increases since it is 26 times(Example 1) to 96 times (Example 1) higher at 15 T.

As is apparent from FIG. 1, any of copper samples (Comparative Examples3 to 6) shows a right downward curve and, as the intensity of themagnetic field increases, the magnetoresistance effect increases ascompared with Comparative Example 2. Namely, it is found that, in caseof copper, a decrease in conductivity due to magnetoresistance cannot besuppressed even if the purity is increased to 6N level (as is apparentfrom Table 1, in samples of Comparative Examples 3 to 6, the resistivityat 15 T increases by 5 to 18 times as compared with the resistivity incase where the magnetic field is absent), and that the effect capable ofsuppressing a decrease in conductivity in the magnetic field byincreasing the purity to 99.999% by mass or more, found by the presentinventors, is peculiar to aluminum.

The reason why, the magnetoresistance suppression effect by highlypurification is not exhibited in copper but is exhibited in aluminum, isunclear. However, it is deduced that it is caused by a difference inelectrical resistivity factor. Namely, it is considered that a mainelectrical resistivity of the high purity copper is the scattering ofconduction electrons due to grain boundaries or dislocations, and theelectrical resistivity factor slightly varies even by highlypurification, and thus magnetoresistance also slightly varies. On theother hand, a main electrical resistivity factor of the high purityaluminum is the scattering of conduction electrons by impurity atoms,and the electrical resistivity factor is decreased by highlypurification. Therefore, it is considered that excellent characteristicssuch as little increase in electrical resistivity in the magnetic fieldmay be exhibited in aluminum having a purity of 5N or more. However,this predictable mechanism does not restrict the scope of the presentinvention.

Then, the thermal conductivity of each sample was calculated from theresults of Table 2.

The results of Table 2 and the results the residual resistivity ratioRRR calculated from the above-mentioned equation (3) are shown in Table3. The value (i.e., resistivity at 4.2 K) in Table 2 was used as ρ_(T)of the equation (3). As mentioned above, ρ_(297 K) is scarcelyinfluenced by the purity and the magnetic field applied from the outsidein copper and aluminum, and is almost constant and can be treated as agiven value in the high purity metals. Therefore, 2,753 nΩcm was used asρ_(297 K) of aluminum and 1,500 nΩcm was used as ρ_(297 K) of copper.

TABLE 3 RRR 0 T 1 T 2 T 3 T 4 T 6 T 8 T 10 T 12 T 15 T Example 3 825610582 10558 11208 10861 10606 11057 10828 10281 9631 Example 2 7795 93589439 9238 9272 9099 8980 8653 8386 7165 Example 1 3829 2642 2707 26302697 2688 2606 2609 2627 2592 Comparative 292 164 134 123 117 111 107105 103 101 Example 2 Comparative 24 23 22 20 19 17 15 14 13 12 Example1 Comparative 957 419 317 279 257 235 223 215 209 202 Example 3Comparative 500 246 150 115 88 68 54 43 37 28 Example 6 Comparative 455205 136 107 83 63 49 43 37 28 Example 5 Comparative 326 167 115 88 71 5444 38 33 27 Example 4

Then, thermal conductivity was calculated using the value of RRR inTable 3, and the equations (1) and (2).

FIG. 2 is a graph showing a relation between the thermal conductivityand the applied magnetic field (magnetic flux density).

As is apparent from FIG. 2, when the intensity of the strong magneticfield in all Comparative Examples, including Comparative Example 2corresponding to a thermal conductor made of a conventional aluminum (4Nlevel) and Comparative Example 6 corresponding to a thermal conductormade of a conventional copper (6N level), increases, thermalconductivity decreases. At the magnetic flux density of 15 T, thethermal conductivity is only at 1,238 W/m/K even in case of ComparativeExample 3 which exhibits the highest thermal conductivity amongComparative Examples.

To the contrary, in Examples 1 to 3, a decrease in thermal conductivityis suppressed even if the intensity of the magnetic field increases.

In Example 1, the thermal conductivity is stable until 15 T afterdecreasing at 1 T, and high thermal conductivity (about 9,500 W/m/K) isexhibited even at 15 T.

In Example 2, thermal conductivity increases in a range from 1 T to 12 Tas compared with the case where the magnetic field is not applied, andhigh thermal conductivity (about 25,000 W/m/K) is exhibited even at 15T.

In Example 3, thermal conductivity increases in a range from 1 T to 15 Tas compared with the case where the magnetic field is not applied, andvery high thermal conductivity (about 33,000 W/m/K) is exhibited even at15 T.

Using the thus obtained thermal conductivity, a temperature difference,generated at both ends of the sample when one end of the sample isconnected to a refrigerator and a heat input is applied to the otherend, was calculated.

More specifically, a temperature difference, generated between both endswhen one end of a sheet-shaped thermal conductor measuring 100 mm inwidth w, 400 mm in length L and 0.5 mm in thickness is connected to acooling stage of a refrigerator cooled to about 4 K and a heat input Qof 2 W is applied to the other end separated by 400 mm, was calculated.

The temperature difference ΔT between both ends was determined by theequation (5).

ΔT=Q×(L/1,000)/(w/1,000)/(t/1,000)/λ  (5)

where

Q: Heat input (W)

L: Length of sheet-shaped sample (mm)

w: Width of sheet-shaped sample (mm)

t: Thickness of sheet-shaped sample (mm)

λ: Thermal conductivity (W/m/K)

FIG. 3 is a graph showing a relation between the temperature differenceof both ends and the magnetic field (magnetic flux density) of the thusobtained sheet-shaped sample. The temperature difference of the ordinatewas indicated by logarithm because of a large difference between samplesof Examples and sample of Comparative Examples.

A temperature difference is scarcely recognized in Examples 1 to 3.ΔT=1.7 K even at 15 T in Example 1, ΔT=0.6 K in Example 2, and ΔT=0.5 Kin Example 3.

To the contrary, in any of Comparative Examples, as the intensity of themagnetic field increases, ΔT also increases. Also in Comparative Example3 in which ΔT at 15 T is the smallest among Comparative Examples, ΔT is13 K. ΔT of Comparative Example 2 corresponding to a thermal conductormade of a conventional aluminum (4N level) is 42 K.

Moreover, these values are values obtained without taking a temperaturedependence of the thermal conductivity λ into consideration, and ΔTfurther increased in case of taking the temperature dependence intoconsideration.

In such way, when using the thermal conductor according to the presentinvention, which has high thermal conductivity even at cryogenictemperature under a strong magnetic field and exhibits excellent heattransfer properties, the cross section can be decreased as compared witha conventional thermal conductor. Therefore, miniaturization and weightsaving of an apparatus including a superconducting magnet can beperformed.

According to the present invention, it is possible to provide a thermalconductor having excellent heat transfer properties by high thermalconductivity even at low temperature of, for example, a liquid nitrogentemperature (77 K) or lower, especially a cryogenic temperature of 20 Kor lower in a strong magnetic field of a magnetic flux density of 1 T ormore.

The present application claims priority based on Japanese PatentApplication No. 2011-101767. The disclosure of Japanese PatentApplication No. 2011-101767 is incorporated by reference herein.

1. A thermal conductor to be used at low temperature of 77 K or lower inthe magnetic field of a magnetic flux density of 1 T or more, comprisingaluminum having a purity of 99.999% by mass or more and having thecontent of iron of 1 ppm by mass or less.
 2. The thermal conductoraccording to claim 1, wherein the aluminum has a purity of 99.9999% bymass or more.
 3. The thermal conductor according to claim 1, wherein thealuminum has a purity of 99.99998% by mass or more.
 4. The thermalconductor according to claim 1, wherein the aluminum contains anintermetallic compound Al₃ Fe.
 5. A thermal conductor for cooling asuperconducting magnet, using the thermal conductor according to claim1.